Our evaluator currently catches and signals only two kinds of errors -- unknown expression types and unknown procedure types. Other errors will take us out of the evaluator read-eval-print loop. When we run the evaluator using the register-machine simulator, these errors are caught by the underlying Scheme system. This is analogous to the computer crashing when a user program makes an error. It is a large project to make a real error system work, but it is well worth the effort to understand what is involved here.
a. Errors that occur in the evaluation process, such as an attempt to access an unbound variable, could be caught by changing the lookup operation to make it return a distinguished condition code, which cannot be a possible value of any user variable. The evaluator can test for this condition code and then do what is necessary to go to
. Find all of the places in the evaluator where such a change is necessary and fix them. This is lots of work.
b. Much worse is the problem of handling errors that are signaled by applying primitive procedures, such as an attempt to divide by zero or an attempt to extract the
of a symbol. In a professionally written high-quality system, each primitive application is checked for safety as part of the primitive. For example, every call to
could first check that the argument is a pair. If the argument is not a pair, the application would return a distinguished condition code to the evaluator, which would then report the failure. We could arrange for this in our register-machine simulator by making each primitive procedure check for applicability and returning an appropriate distinguished condition code on failure. Then the
code in the evaluator can check for the condition code and go to
if necessary. Build this structure and make it work. This is a major project.
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